1. Field of the Invention
This invention relates to radiation-sensitive coating compositions, processes for the preparation of a printing plate using such coating compositions, and printing plates prepared in such a manner. In particular, this invention relates to such compositions as well as printing plates having a radiation sensitive layer containing homogeneously distributed polymer particles. Printing plates employing such a photosensitive layer exhibit high print run stability.
2. Background Information
In recent times, the requirements with respect to the print run stability of offset printing plates have increased. For the improvement of the mechanical properties of offset plates, basically two methods have been disclosed.
One method relates to the improvement of the properties of the binders or the photosensitive composition (U.S. Pat. No. 4,102,686). This method often entails high costs due to the complexity of the synthesis regarding the preparation of such custom high-performance binders.
The other, less expensive method is the introduction of polymer particles into the radiation sensitive layer. The mechanical resistance of such particle-containing layers is often superior to that of layers with modified binders. Apart from the increase of the mechanical resistance of the layer, there are further advantages: From the literature, it is known that the chemical resistance can be increased while the exhausting times in the copying frame and the adhesiveness can be reduced. Thus far, various methods have been employed for introducing the polymer particles into the layer.
Commercially available micronized particles which are difficult to dissolve in common organic solvents (for example polyethylene, polytetrafluoroethylene, cross-linked polymethylmethacrylate or polyamide) are typically dispersed and added to the coating solution. This entails the disadvantage that these polymer particles first have to be dispersed, which constitutes an additional process step. In some areas, the particles are often distributed rather inhomogeneously after the coating has been completed. Furthermore, for particles consisting of nonpolar polymers, the adhesion to the layer is reduced. The retention time of the coating solution prepared is limited as well since these dispersions tend to form sediments or agglomerates. Furthermore, the particle dispersions cause problems in the filtering of the coating solution since the pore size of the filter cannot be smaller than the particle diameter. Starting solutions which have not been sufficiently filtered often lead to coating defects.
Another method is to prepare dispersions or emulsions of polymers by emulsion/dispersion polymerization, for example in copolymers of acrylates, styrene derivatives and other monomers, or by precipitation reaction of polymers which dissolve in common organic solvents, for example acrylonitrile butadiene styrenes (ABS) which are then added to the coating solution (U.S. Pat. Nos. 4,233,390 and 5,616,449, EP-A-0709 399,, EP-A-0 632 328, EP-B-0 509 523).
Thus, due to the higher polarity of the polymer particles, an improved adhesion of the particles to the layer is achieved, but the above-mentioned disadvantages such as poor filterability, locally inhomogeneous distribution in the layer, and short stability of the batch remain.
Another process describes, with regard to positive working printing plates, the use of specifically developed polymers which are soluble in alkaline developers and organic media and which form a homogenous solution with the coating composition. Using a complicated drying apparatus which is difficult to operate from a process control standpoint, a separation of the binders intended for the positive plates and the polymers during drying is achieved (EP-A-0 766 140). This coating solution can be filtered since a xe2x80x9crealxe2x80x9d solution is present. A disadvantage of this method is that the particles are preferably formed in the upper part of the photosensitive layer, with the lower part of the layer not being protected from mechanical abrasion. Furthermore, both the drying apparatus and the polymers which form the particles have to be specifically adapted to this purpose of use, which constitutes a high cost factor. A further disadvantage of this method is that the resulting polymer particles, which are soluble in the developer, can partly be extracted from the layer by the developing step and that the remaining cavities destabilize the image areas.
In spite of the intensive research in the field of offset printing plates with high print run stability, existing approaches still require improvements, particularly with respect to easier producibility and less expensive materials.
U.S. patent application Ser. No. 09/493,837 discloses a coating composition for producing printing plates containing particles of a thermoplastic polymer in the radiation sensitive layer. The coating solution comprises a conventional positive, negative or electrophotographically radiation sensitive composition, a thermoplastic polymer soluble in organic media but insoluble in alkaline media, and two solvents which show different solubilizing properties for the thermoplastic polymer. Although the plates obtained according to this disclosure show a great improvement over the other known plates, further improvement is still desirable.
Therefore, it is the object of this invention to provide coating compositions which allow the rapid preparation of printing plate precursors, particularly lithographic printing plate precursors showing high print run stability. Additionally, the printing plate is capable development using conventional developers and exhibits high resolution. Furthermore, the ink acceptance of the printing plate is rapid and superior. Moreover, a sufficient resistance to the common purification chemicals such as benzine-based rubber blanket detergents, plate detergents etc. is provided.
The coating solution of this invention is useful in the preparation of printing plate precursors, and comprises:
a) a radiation sensitive composition C comprising a phenolic resin;
b) at least one thermoplastic polymer P which has a solubility in aqueous alkaline media ranging from sparingly soluble to insoluble, and preferably is substantially insoluble in aqueous alkaline media;
c) a first solvent component A which is capable of solubilizing composition C and thermoplastic polymer P;
d) a second solvent component B having a volatility less than component A, wherein component B is capable of solubilizing composition C but not thermoplastic polymer P, and composition C and thermoplastic polymer P are homogeneously dissolved in a mixture of components A and B; and
e) at least one further polymer AP having a higher molecular weight than the phenolic resin of composition C, wherein polymer AP is miscible with the phenolic resin and immiscible with thermoplastic polymer P.